Semiconductor etching, cleaning, and deposition processes typically employ a plasma mediated process that is desirably carried out at a reduced pressure, e.g., in an evacuated (vacuum) chamber. It is important to maintain the pressure within the chamber within a specific predetermined range in order to avoid costly delays in the semiconductor wafer production process and to minimize undesirable variations in the quality of the semiconductor wafer products that are produced. Maintaining pressure within the predetermined range is difficult since, during device fabrication, substrates are sequentially fed into the processing chamber in a continuous or batch process from an external source operating at atmospheric conditions. Time spent controlling and readjusting the chamber pressure for each substrate or substrate batch introduced into the processing chamber can greatly increase processing times. The decreased throughput resulting from controlling and readjusting the pressure increases overall device costs.
Chamber overhead time is defined as the time required for any operation involving the process chamber that does not include actual wafer processing time. The process chamber overhead time typically includes the time period for reducing the pressure within the process chamber to the desired processing pressure after each wafer exchange, heating the wafer to the desired temperature, venting the process chamber to allow wafer exchange and the wafer exchange itself. Minimizing overhead time increases productivity and reduces overall device costs.
Numerous apparatuses and methods exist for transferring semiconductor wafers into or out of a process chamber for continuous treatment without disturbing or otherwise affecting chamber pressure. Many such devices teach the use of an airlock chamber, i.e., a loadlock chamber, in operative communication with the processing chamber. Such a loadlock chamber can be adjusted to match the operating pressure in the processing chamber, thereby allowing transfer of substrates into or out of the process chamber while also allowing the process chamber to maintain a relatively constant pressure. In these devices, robots are generally implemented as a single arm whose travel moves a wafer in a substantially linear manner. The arm translation path is configured such that a central axis of the wafer passes over or near a central robotic arm pivot. Such pivots are typically mounted in the center of the loadlock chamber due to physical size limitations imposed by robotic link arm design and associated link arm travel. As a result, these types of transfer mechanisms suffer from excessive internal chamber volume in the loadlock chamber assembly due to the required translating arm paths of the transfer mechanism. Moreover, since the primary or first pivot of the link arm is centrally located within the loadlock chamber, repair and access to the apparatus is difficult. Also, the prior art often uses a complex system of a timing belt and pulley arrangement coupled to a step motor drive output shaft, and a sleeve coupled to a first link arm axis, in order to effect rotation of the arms.
For example, U.S. Pat. No. 4,584,045 to Richards, discloses the use of a belt drive in a wafer positioning transport apparatus. A problem exists through the use of a spring in one of the arms of the transfer mechanism. As the belt wears or stretches, the spring extends the arm to keep the belt tight. This alters placement of the semiconductor wafer in the chamber. Wafer positioning devices necessarily must be very accurate in the positioning at all stages of operation of the device. Such wear, which alters placement, is undesirable.
In U.S. Pat. No. 4,728,252 to Lada, a complex wafer transport mechanism is disclosed. The device of this patent has one shaft sealed within another shaft, which rotates independently of the outer shaft. A complex seal mechanism inherently exposes the device to potential failure and fretting. Also, the device employs belts and requires two motors and two motor control circuits, with the attendant wire harness and the like. The complexity of this device makes it expensive. Moreover, the use of belts increases the potential for fretting or wear, producing contaminants. In addition, as the belts wear or stretch, they need to be replaced on a regular basis, both in order to maintain the accuracy of the operation of the device, as well as to keep the number of contaminating particulates down within the apparatus. Replacement of belts produces additional maintenance costs and undesirable down time for the system.